Cinnamic acid derivatives are important commercial materials, with diverse uses such as, for example, antioxidants in plastics, ultraviolet absorbers, pharmaceutical intermediates and the like. Several cinnamate esters find applications as ingredients in sunscreen formulations. See, for example, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-Interscience, New York, Vol. 7, page 153 (1979). An example cinnamate ester that has high commercial value is 2-ethylhexyl p-methoxycinnamate (EHMC) (Formula 1). ##STR1##
Processes to synthesize cinnamic acid derivatives are known. For example, a process to make EHMC involves coupling of an arenediazonium salt with an unsaturated compound under palladium catalysis. A review of this approach is provided by R. F. Heck, Palladium Reagents in Organic Synthesis, Academic Press, London, 287-290 (1985).
U.S. Pat. No. 4,970,332 discloses a process to make EHMC wherein p-anisidine is first diazotized and converted to p-iodoanisole. p-Iodoanisole is then reacted with 2-ethylhexyl acrylate in the presence of a trialkylamine and a palladium catalyst. EHMC and trialkylaminehydroiodide are produced in the reaction which are then separated.
Other pertinent publications in this area include K. Kikukawa et al, J. Organometallic Chem., Vol. 270, pages 277-282 (1984), which describes coupling of arenediazonium tetrafluoroborates with vinylsilanes using palladium catalysts. XU Jian-Hua et al, Organic Chemistry (Youji Huazue), No. 6, pages 452-454 (1987) describe Li.sub.2 PdCl.sub.4 --CuCl catalyzed coupling of arenediazonium tetrafluoroborates with methyl acrylate in methanol to yield cinnamic acid derivatives.
A problem with such coupling reactions of arenediazonium compounds with alkenes is the poor yield of the desired cinnamic acid derivatives. Generally in these reactions, an acid such as, for example, tetrafluoroboric acid is formed as a byproduct. This acid then polymerizes the alkene reactant to polymeric products. This side action reduces the availability of the alkene for the desired coupling reaction.
Due to the commercial significance of cinnamic acid derivatives, there is a continuing interest in overcoming the above-mentioned problem as well as in discovering new synthetic processes to prepare them economically and from readily available ingredients, in high yields and purity.